1. Field of the Invention
This invention generally relates to prosthetic implants having a porous surface attached thereto and more particularly to improving the fatigue strength of a such an implant.
2. Description of Related Art
Prosthetic implants for the replacement of a portion of a patient""s joints are well-known, and may be constructed of cobalt-chromium-molybdenum or titanium, for example. Similarly, it is known to provide a porous surface layer on the implant to promote fixation by allowing direct bone ingrowth and interdigitation with the implant. Alternatively, the porous surface may receive bone cement therein to enhance the mechanical interlock with bone cement. The porous surface layer typically takes the form of a plurality of small metallic particles such as beads or a wire mesh. Commonly, the porous surface layer is sintered, diffusion bonded, or welded to the implant. These processes require heating the implant and particles to a temperature sufficient to cause the porous surface layer and implant body to fuse, melt or bond together at their point of mutual contact.
A phenomenon with beaded and/or other textured surfaces is that the texturing creates a xe2x80x9cnotch effectxe2x80x9d on the surface of the implant. If the bonded junctions were viewed in cross section, a small notch would be seen extending into the implant on each side of a contact point between the porous surface layer and the implant. This so-called xe2x80x9cnotch effectxe2x80x9d contributes to crack formation when the implants are cyclically loaded in a fatigue mode.
A related phenomenon with beaded or textured surfaces is that the sintering process by which the beads are typically adhered to the implant creates an annealing effect which reduces the fatigue strength of the implant. This annealing effect is particularly noticeable in forged implants which have a higher fatigue strength, due to working of the metal, than their cast counterparts before bead bonding.
U.S. Pat. No. 5,443,510, assigned to the assignee of the present invention and hereby incorporated by reference, discloses addressing the xe2x80x9cnotch effectxe2x80x9d phenomenon by reducing the number of notches formed. The formation of notches in the implant body can be limited by creating a thin layer of metal mesh on the surface of the implant and then bonding the porous surface layer onto the mesh.
U.S. Pat. No. 5,734,959, assigned to the assignee of the present invention and hereby incorporated by reference, discloses a method for enhancing the bonding between the porous surface layer and the implant. An organic binder such as a water-soluble protein is used to enhance the bonding between the porous surface layer and the implant. During the sintering process, the binder carbonizes and alloys with the metal of the porous surface layer and thereby lowers the melting temperature of the metallic particles at the interface surfaces. Other alloy materials such as silicon can also be suspended in the binder with this method. This patent does not address the notch effect phenomenon.
U.S. Pat. No. 5,308,412, assigned to the assignee of the present invention and hereby incorporated by reference, discloses a method for surface hardening cobalt-chromium based orthopaedic implants by a nitriding or nitrogen diffusion hardening process. The ""412 patent is aimed at increasing the wear-resistance properties of the surface of the implant so as to reduce the wear debris produced from articulation against polyethylene, metal, or ceramic counterfaces or by micro-motion of the implant relative to the environment contacting the implant, typically bone or bone cement. The ""412 patent suggests that the nitriding process disclosed therein results in minimal or no loss of fatigue properties to the implant.
The present invention provides an implant having enhanced fatigue strength by incorporating a substance into the implant which reduces the melting point of the substrate prior to sintering the porous layer to the substrate. In so doing, sintering can be performed at a lower temperature, which in turn significantly reduces the fatigue strength loss from a forged implant which occurs during the sintering process.
The present invention also provides a nitriding process and thermal processes to which the implant can be subjected after the sintering process is completed. The nitriding or nitrogen diffusion hardening process and the thermal processes further increase the fatigue strength of a cast or forged implant.
In one form thereof, the present invention provides a method for forming a porous layer on a forged orthopaedic implant. First, an orthopaedic implant substrate formed from a forged metal alloy and having a surface adapted to support a porous layer and a plurality of metallic particles are provided. A substance is incorporated into the forged substrate which substance reduces the melting point of the substrate. The substrate surface and the metallic particles are brought into contact with one another and heated to a temperature less than the reduced melting point, whereby the particles bond to the surface. In a preferred form, the forged alloy is cobalt-chromium-molybdenum alloy. The melting point lowering substance can be carbon, silicon, nitrogen, niobium, columbium, tantalum, chromium carbides, chromium nitrides, chromium silicides, molybdenum silicides, chromium borides, silicon carbides, silicon nitrides, titanium carbides, titanium aluminides, titanium silicides, zirconium carbides or zirconium silicides.
One advantage of the method described above is that it compensates for the xe2x80x9cnotch effectxe2x80x9d and the reduction in fatigue strength which results from the high temperatures and long times involved in sintering. That is, the method in accordance with the present invention provides a forged implant which maintains most of its fatigue strength through the sintering process.
In another form thereof, the present invention comprises a method for increasing the fatigue strength of an implant having a porous layer thereon. The implant can be formed from either a cast or forged material. An implant substrate formed from a metal alloy and having a surface adapted to support a porous layer and a plurality of metallic particles are provided. The metallic particles are brought into contact with the substrate surface. The metallic particles and implant substrate are heated to a temperature sufficient to sinter the particles to the surface, whereby the particles bond to the surface and form a porous layer. Then, the implant is gas quenched down to at least room temperature. The substrate is then heated to an aging temperature range of about 800xc2x0 F. to 2100xc2x0 F. and aged at the aging temperature for 1 to 100 hours.
In a preferred form, the method includes gas quenching the implant to below xe2x88x9290xc2x0 F. or between xe2x88x9290xc2x0 F. and xe2x88x92300xc2x0 F. during the gas quenching step.
One advantage of these thermal processing methods is that they can be used in addition to or separately from the method of incorporating a melting point lowering substance into the substrate.
Another advantage of the inventive thermal processing methods is that they can be used to enhance the strength of forged or cast parts, and can be used with porous coated or uncoated implants.
In yet another form, the present invention provides a method of increasing the fatigue strength of a beaded implant. The implant can be forged or cast. A beaded orthopaedic implant substrate is provided and then exposed to an atmosphere of molecular nitrogen gas or atomic nitrogen at a process temperature within the range of 500xc2x0 F. to 2400xc2x0 F. for a process time duration sufficient to achieve increased fatigue strength.
An advantage of the inventive nitriding process in accordance with the present invention is that it significantly improves the fatigue strength of a cast or forged implant. Thus, cast or forged implants subjected to sintering can have their fatigue strength restored by subsequently using the nitrogen diffusion hardening or nitriding process in accordance with the present invention.
Another advantage of the inventive methods of the present invention is that they can improve the mechanical properties of a wide variety of implants, such as for hip, knee, shoulder, elbow and other joints.